Electrolytic hydrogen is usually produced by electrolysis of an aqueous solution at an operating voltage of 2.0 V. During this process, oxygen is also produced, making the separation of hydrogen and oxygen necessary. The high operating voltage necessary for the electrolysis of water is due to the high energy needed for breaking the OH bond, which is reasonably strong. Several attempts have been made to reduce the operating voltage of the electrolysis by addition of various anodic depolarizers like coal, biomass, etc., to the eletrolyte.
The chemical bond strength of H-O is 102.3 kcal mol.sup.-1. In H.sub.2 S, the bond strength of H-S bond is only 82.3 kcal mol.sup.-1. Hence, the basic thermodynamically reversible potential for electrolysis of an aqueous solution saturated with H.sub.2 S should be less than that of water.
For the overall reaction EQU H.sub.2 S.fwdarw.H.sub.2 +S (1)
the standard potential is 0.171 V compared to 1.23 V for the reaction EQU H.sub.2 O.fwdarw.H.sub.2 +1/2O.sub.2 ( 2)
The free energy change for the reaction (1) is 7.892 kcal mol.sup.-1. The energy needed to break the HS bond and produce hydrogen is not high enough to produce oxygen at the anode, and thus avoids any separation in the electrolytic processes. Further, when hydrogen is produced by breaking the HS bond, (elemental) sulfur is produced.
A prior art of hydrogen sulfide removal is disclosed in U.S. Pat. No. 3,409,520. In this, it is suggested that a hydrogen-sulfide-hydrocarbon gas mixture is introduced into the electrolysis cell having an electrolyte operating at room temperature. The gas mixture comes in contact with porous electrodes activated with platinum catalysts. An externally generated current is passed through the cell. The sulfur produced at the anode blocks the reaction, which is subsequently removed by circulating a solvent to reactivate the anode. In this prior art, the cell will lose the efficiency with time due to the poisoning of the catalyst by sulfide ions accumulated in the solution as a result of the continuous dissolution of the hydrogen sulfide gas.
In another prior art disclosed in U.S. Pat. No. 4,544,461, a catalytic anode material is described for hydrogen sulfide decomposition. This again operates at room temperature and uses a solvent for frequent removal of sulfur formed at the anode.
One drawback of the prior art of the electrolytic decomposition of H.sub.2 S is the use of catalyst that is normally poisoned by hydrogen sulfide. Unfortunately, the poisoned catalyst will not be effective in further decomposition of H.sub.2 S and will consume a larger amount of energy to decompose further amounts of H.sub.2 S. Since the catalysts used are based on noble metals, the process will be expensive.
A second drawback in both the prior arts described here is the use of a special solvent for removal of deposited sulfur from the anode. This hampers the continuous operation of the cell, reduces the efficiency of the electrolytic process (with gradual build-up of sulfur at the anode) and possible contamination of the electrolyte with the solvent used for removing deposited sulfur.
The serious drawbacks of the earlier inventions have been completely eliminated in the invention disclosed here.